When I adust the 1 and 3 valves there is a notch on the impeller. Wouldn't TDC for valves 2 and 4 be at 180 degrees from the the notch on the impeller? I could paint the notch white and the other side yellow and have someone look for the paint while I spin the tire?

you can also look at the position of the distributor rotor. there is a notch along the perimeter of the dizzy housing and when the rotor points to that notch, the engine is at TDC for cylinder #1. rotate engine so rotor points 90 degrees in rotation, that's TDC for another cylinder, rotate another 90, TDC for another and one more time for the final cylinder. i "think" the rotation is clockwise to go in the firing order 1-4-3-2 to bring the cylinders 1-4-3-2 in turn to TDC.

The Pelican Parts Tech article on valve adjustment suggests exactly that. By lining up the notch at the top of the case (use the mirror or your finger to line it up where the case and tranny bell housing meet at the top by the rear fire wall) you can then crawl under the car and make a mark at the case split line. I used a dremel because I don't trust the paint to stick over time.

Once you do this you can take all your stuff with you under the car and not have to crawl back out until it's all done.

If you read the PP article mentioned by Herb, the end of the article has an alternate valve adjustment method that doesn't rely on TDC. I find it much easier. It simply works off the principle that when two valves are depressed by the high side of the cam lobe, the opposing valves are closed and can be adjusted. And you don't have to get up & down, or rely on some else checking TDC marks for you.

I don't use the "rocking" method mentioned by Gerard. I think it's a little too likely that I will wind up trying to adjust one valve while its neighbor on the same cylinder is open. If you do that, the clearance will be wrong. BT, DT, on several engines. (Set the clearance corretly, then check it again on that valve when the other valve on that same cylinder is open. Betcha you'll find a larger clearance...)

For me, the KISS principle dictates using the TDC method. I find it harder to screw that one up. For someone else, it may dictate the other method.